Answering the Fuel-cell Compressor Question

November 1, 2020

Lindsay Brooke

What’s the optimum way to feed air to a hydrogen fuel cell? Since industry interest in fuel-cell (FC) propulsion for ground vehicles was kindled in the 1990s, development engineers have investigated various solutions to supply air to the cathode (input) side of fuel-cell stacks. Prototype stacks have employed Roots-type compressors, centrifugal machines, scroll- and twin-screw-type devices and even miniature Wankel rotary compressors (part of a Ballard Power Systems patent in the early 2000s).

Each technology has benefits and disadvantages, depending on the specific power strategy and application. Eaton Roots-type superchargers have been present on FC development programs at many OEMs. Since then, there has been a bifurcation in the market as industry focus has expanded beyond light passenger vehicles to heavy-duty truck applications, according to Dr. Mihai Dorobantu, director, technology planning and government affairs at Eaton’s Vehicle Group.

“We’re seeing centrifugal devices used by Bosch and Garrett-Honeywell, which are smaller and quieter than the Roots device – but for efficiency reasons they need to spin at 120,000 rpm to 150,000 rpm. Really high speed!” Dorobantu told SAE International.

Eaton sees the thrust of fuel-cell vehicle applications currently directed to commercial vehicles, where hydrogen can be a serious competitor to diesel fuel in terms of energy density, refueling time and overall cost to move one ton of freight one mile.

“There is a debate within the FC development community over compressor technology: Should it be a Roots-like low-speed device in which the speed of the rotor – rated at 10,000 to 20,000 rpm – controls the exact amount of air that is being pushed into the fuel cell?” Dorobantu asks. “Or should it be a high-speed centrifugal device that’s based on turbocharger technology? There are different philosophies in the fuel-cell business.”

The air-delivery challenge

Eaton has a significant development program that is tailoring the company’s Twin Vortices Series (TVS) Roots-type supercharger, widely employed in high-performance V8 combustion engines as well as in a family of diesel EGR pumps, for electrically-driven FC duty. The TVS design uses a pair of four-lobe rotors, each twisted 160 degrees, to provide a higher helix angle. In a V8, the TVS design is proven to deliver 12% greater efficiency, with lower noise and vibration compared with a conventional Roots-type lobe geometry.

Eaton’s TVS design is available in two different specifications: the four-lobe R-Series that is optimized for peak power applications and the three-lobe V-Series aimed at low-flow efficiency, particularly for two-stage boosting systems. But compared with the blowers used on muscle-car engines, the Eaton compressors aimed at FC applications have different lobe geometries, lower-inertia reciprocating components and very different operating requirements, Dorobantu explained.

“The fuel cell unit has to deliver air in very precise quantities and the way this is set up, it operates in high-, mid- and low-load situations. But the question is, where do you optimize for peak efficiency? And how does your efficiency degrade when you’re off the peak?

“Superchargers are designed for maximum flow for when you need snap acceleration and the extra horsepower,” he continued. “It doesn’t need to be efficient at low speeds or low-flow situations. In a fuel cell, because it is used at different loads, it becomes really important that the air-supply device is efficient across its full operating range. Because if you’re not using the device efficiently, you’re just burning hydrogen.”

Some fuel cells are designed for optimal efficiency at one operating point (no transients) – part of a “charge maintaining” strategy that constantly recharges the vehicle battery, which is larger. Centrifugal machines might make sense in this scenario. But Dorobantu argues that they have surge and choke lines “so you have to be careful when you start them up – navigate between those lines, get to your operating point and stay there forever.”

“Load-following” FCs, by comparison, are sized to maximum demand and are paired with a small battery. The fuel cell follows demand and can operate efficiently at any point, “so you don’t end up with a stalled, or choked or surged compressor,” he said. For load-following FCs, the Roots-based TVS compressor is well-suited.

In heavy commercial-vehicle applications, FC peak power typically is around 300 kW, but the average power is 150 kW. Adding more stack costs less than adding more battery. “In the big-stack/small-battery model, the air-supply system must be able to follow load and be efficient at almost all load situations,” he explained. “On the other hand, in the charge-sustaining strategy (small stack/big battery), it is important to have the most efficiency possible on that one operating point.”

Going forward, Eaton’s TVS development will continue to explore lightweight materials for faster response; higher pressure ratios (up to 3:1) to increase efficiencies across the operating map; reduced noise and vibration, and energy recovery.

“The air compressor is the largest parasitic on a fuel cell,” Dorobantu said. “For example, for an 80-kW fuel cell, you can expect 10 kW or more to go into the [compressor’s] electric motor. Working with the DoE [U.S. Dept. of Energy] a few years back, we showed that we can recover 5 kW of parasitics at the highest load. There is still a lot of technology needed for these machines to achieve their mission.”

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